California Palm Trees & Beetles

Ornamental California fan palm trees (Washingtonia filifera), grown in U.S. Department of Agriculture plant hardiness zones 8 to 11, are evergreens that produce shade and food for people and wildlife. Palms, which attract a number of garden variety pests, are hosts to giant palm borer (Dinapate wrightii) beetles that feed on trunks and limbs. Red palm weevils (Rhynchophorus ferrugineus), first discovered in the United States in 2010, are also a threat to California palms.

Adult giant palm borers are brownish-black beetles about 1 to 2 inches long. Females chew holes from 1 to 2 inches wide into palm tree trunks. They lay eggs after mating with male beetles. Larvae feed on the trunk wood for a few years and, at maturity, the adults chew their way through the tree -- usually at night. A beetle-damaged palm tree has yellow fronds and tunnels that are dug through the inside of the trunk. These tunnels affect the vascular system and weaken the tree, which may die or fall over in strong winds. Removing dying trees and infected plant debris helps to control beetle infestation in neighboring plants.

Originally from Southeast Asia, red palm weevils -- also called RPW -- can kill palm trees. Females drill holes into tree trunks, each laying 50 to more than 500 eggs. Eggs hatch in about three days. Emerging larvae bore inward, damaging the tree’s vascular system so that it cannot transport water and nutrients. After approximately two months, larvae pupate in the tree for about three weeks, developing into adult beetles. Growing 1 to 2 inches long, adult RPWs live for two to three months, feeding on palm leaves, shoots and limbs. RPWs mate several times in their life cycle, laying more eggs. Insecticides may control RPW infestations, notes the University of California’s Center for Invasive Species Research. Burying, burning, chipping and removing dead, beetle-infested palm trees helps to control future outbreaks.

California fan palm trees and other varieties may attract American palm cixiid (Haplaxius crudus), which grow to about 1/4-inch long. Symptoms of infested palms include loose or early-dropping fruits with black or brown stems, yellowing fronds and dark flower clusters. Insects eat Lethal Yellowing-infected palms, also called LY disease, and then move to healthy trees. Severely damaged trees fall within three to six months. Coconut Rhinoceros Beetles (Oryctes rhinoceros), native to Southeast Asia, bore holes into palm tree crowns and fronds. Larvae eat leaf debris and rotting wood. Reddish-black adult beetles, which grow nearly 2 inches long, eat plant juices at night. Males and females have single horns and live four to five months. Adult beetles drill into palm crowns, causing foliage to die. South American Palm Weevils (Rhynchophorus palmarum) are shiny, dark black insects that chew through trees, causing necrotic flowers and yellowing fronds. Larvae feed on plant tissue.

Many types of California palms feature large, fanned leaves with leaflets growing from single stems. Foliage may be feathery and barrel-shaped trunks are either smooth or rough. Fan palms typically grow from 40 to 60 feet high and spread 10 to 15 feet wide. California palms thrive in a variety of soils, prefer full sun and can tolerate drought and frosts. California palm trees are generally resistant to bacterial disease but can develop Diamond Scale (Phaeochoropsis neowashingtoniae), Fusarium Wilt (Fusarium oxysporum), Pink Rot (Nalanthamala vermoeseni) and Sudden Crown Drop, when leaf crowns and trunk parts fall unexpectedly. California palms attract a variety of other insects in addition to beetles and weevils, including aphids, scales, thrips, mealybugs and spider mites.